Iain M. Banks - Feersum Endjinn

All is not well in the mammoth, multitiered underground city-state of Serehfa, where the king and his clan are waging an inexplicable battle with the engineer clan. Meanwhile, the entire planet anxiously awaits the arrival of a dust cloud headed for the sun--a development called the encroachment that threatens to plunge Earth into a life-extinguishing ice age. Having abandoned long ago the means and expertise to flee into space, humanity's only hope for technological deliverance is the crypt, a ubiquitous computer mainframe that stores all recorded knowledge, including the downloaded minds of the dead, but which has been almost totally corrupted by viral chaos. Defying the king's bewildering lack of concern for the encroachment, a rebel scientist, a dead officer living on in the virtuality of the crypt, and a semiliterate youth try to penetrate the crypt's chaotic levels and retrieve the needed knowledge before it's too late.